Water temperature of heated water at an outflow point such as a bath or shower outflow is conventionally established by throttling and mixing separate hot and cold water flows to a common discharge. When a steady state flow rate occurs that has an acceptable temperature at the outflow, the desired discharge is acceptable.
Everyone who is familiar with a bath or a shower knows that this water mixing has its drawbacks. Hot water flow normally occurs from a remote hot water source, such as a recirculating loop or a heated storage tank containing water heated within a selected temperature range. Cold water flow occurs from an uncontrolled line source--subject to variations in temperature responsive to building temperature and outdoor temperature. Mixing of the hot and cold water flows must be adjusted immediately adjacent the desired outflow point. There are periods of time where of the mixed flow of water is too hot or too cold. These periods of time where the water outflow is too hot or too cold can be extensive. The delay in reaching a desired temperature can come from the start up conditions in the initial mixing--as where the hot water pipes and hot water flow come up to the desired temperature. Alternately, such changes can occur from changes occurring during flow--as where the water pressure changes or the supply of hot water runs out.
Certain devices must rely on a water or fluid outflow at an absolute constant temperature. An example of such a device is that female douche product sold by Andermac Inc. of Marysville, California under the registered trademark "Surgigator.RTM.."
Simply stated, this device is utilized by women to irrigate the vaginal areas usually as a prescribed post surgical treatment. Thermal excursion of the water used for the irrigation is unacceptable. Where the water is too cold, discomfort results; where the water is too hot, a dangerous condition either threatening scalding or causing scalding can occur. Simply stated, water temperature outflow from such a device must be precisely controlled.
Further, and when it comes to the use of such medical instruments, the patients usually are in an isolated water closet undergoing what is essentially a very private proceeding. At the same time, these women patients can have diminished mobility--and sometimes perceptive capacity--to effect the desired mixing. This being the case, the required temperature at the discharge must be precisely and exactly controlled on essentially a default basis with either minimal patient attention or even no patient attention.
Other applications exist where water or fluid temperature must be precisely control]ed. Such applications can include recirculating chemical baths utilized in certain chemical processes--especially those chemical baths utilized for treating electronic products such a micro chips and printed circuit boards. Indeed, it would be desirable to have at many sources a precisely controlled temperature outflow of water or other liquid which did not necessitate the remote heating and mixing of volumes of water. Further, and as to all such outflows, the achieving of steady state flow rates at precise temperatures at outflow points within a small time period is desirable.
So-called "flow through" heaters are known. Specifically, such heaters contain a heating element and channel passing water through the unit in a "single" pass. In a commonly known combination, water is routed about a heater element in a U-shaped flow path. The water is heated in a single pass about the heating element. Water is thereafter directly routed to the flow destination.
Controllers known for such single pass water heaters work on the well known thermodynamic "enthalpy" equation. By understanding the temperature of the water into the heater, the desired temperature of the water out to the source to be heated, and the precise flow rate of the water through the system, temperatures can be controlled.
Controllers using the enthalpy principle have at least two disadvantages. First, all such controllers tend to be subject to "overshoot", that is a tendency at one time or another to exceed the programmed "set point" temperature of the thermostat. In one known device, water heating is controlled by limiting current into the single pass water heater by incrementally adjusting the phase angle input to the single pass electrical heater from alternating current. This device, when initially turned on, commonly overshoots the desired "set point" temperature limit.
Secondly, such devices commonly include flow meters to calculate the energy required for precise temperature control. These flow meters themselves are expensive--making the costs of such devices not practicable.